ArrayBuffer没有直接的合并方法，所以要绕一下

1. 首先把两个ArrayBuffer转换成Unit8Array
2. 然后用一个大的Unit8Array把两个小的Unit8Array装进去
3. 最后把大的Unit8Array转换成ArrayBuffer

/**
 * Creates a new Uint8Array based on two different ArrayBuffers
 *
 * @private
 * @param {ArrayBuffers} buffer1 The first buffer.
 * @param {ArrayBuffers} buffer2 The second buffer.
 * @return {ArrayBuffers} The new ArrayBuffer created out of the two.
 */
var _appendBuffer = function(buffer1, buffer2) {
  var tmp = new Uint8Array(buffer1.byteLength + buffer2.byteLength);
  tmp.set(new Uint8Array(buffer1), 0);
  tmp.set(new Uint8Array(buffer2), buffer1.byteLength);
  return tmp.buffer;
};

参考

https://gist.github.com/72lions/4528834